Considerable interest exists with respect to the subject of sterilization of animals. This is especially true of those concerned with veterinary medicine and animal husbandry, particularly as they relate to the subject of sterilization of domestic animals such as dogs, cats, cattle, sheep, horses, pigs, and the like. Various methods have been developed over the years to accomplish sterilization. For example, with respect to male cattle, the most widely used procedure for eliminating problems of sexual or aggressive behavior is sterilization through surgical castration. This is done in various ways, e.g., crushing the spermatic cord, retaining the testes in the inguinal ring, or use of a rubber band, placed around the neck of the scrotum, to cause sloughing off of the scrotum and testes. However most of these "mechanical" castration methods have proven to be undesirable in one respect or another; for example they (1) are traumatic, (2) introduce the danger of anesthesia, (3) are apt to produce infection, and (4) require trained personnel. Moreover, all such mechanical castration methods result in complete abolition of the testes and this of course implies complete removal of the anabolic effects of any steroids which are produced by the testes and which act as stimuli to growth and protein deposition.
These drawbacks have caused consideration of various alternative sterilization techniques such as the use of chemical sterilization agents. However, the use of chemical sterilization agents has its own set of advantages and disadvantages. On the positive side, chemical sterilization eliminates the stress and danger associated with mechanical castration. Chemical sterilization also has the added advantage of allowing for retention of certain anabolic effects resulting from a continued presence of low levels of circulating testosterone. This is especially valuable in the case of animals raised for human consumption since circulating testosterone promotes growth, efficiency of feed conversion and protein deposition. Unfortunately, there are several disadvantages associated with chemical sterilization. For example chemical sterilization is often temporary rather than permanent; it also sometimes produces extremely severe, and even fatal, side effects.
Many of these chemical sterilization methods have been aimed at regulation of luteinizing hormone produced at various stages of an animal's sexual development. For example, with respect to cattle it has been established that in the case of the infantile calf, luteinizing hormone is rarely discharged and testicular production of androgens is at low levels. On the other hand, in a prepubertal calf, or an adult bull, discharges of luteinizing hormone from the anterior pituitary occur more frequently and the testes produce considerably larger amounts of testosterone and other steroids. It is thought that these conditions result from the following factors: (1) decreases in the concentration of estradiol receptors in the hypothalamus, (2) concomitant increases in the concentration of estradiol receptors in the anterior pituitary, and (3) increases the number of gonadotropin-releasing hormone (GnRH) receptors in the anterior pituitary. This increase in GnRH receptors is generally regarded as a prerequisite for an animal to pass from the infantile stage to the prepubertal and mature stages of endocrine development. Hence, based upon these understandings of the hypothalamic-pituitary-testicular axis, several chemical methods have been proposed to modify given animals, e.g., a bull calf, in such a way that it never enters puberty, but still receives stimuli for growth and protein deposition through the anabolic effects of steroids produced by modified testes. In any event, most of the chemicals proposed for such sterilization purposes are hormones or hormone analogs. For example U.S. Pat. No. 4,444,759 teaches the use of a class of peptides analogous to GnRH (i.e., gonadotropin-releasing hormone, and particularly luteinizing hormone-releasing hormone) are capable of inhibiting release of gonadotropins by the pituitary gland and thereby inhibiting release of the steroidal hormones, estradiol, progesterone and testosterone. It should also be noted that the terms "GnRH" (gonadotropin-releasing hormone) and "LHRH" (luteinizing hormone-releasing hormone) are sometimes used interchangeably in the literature. For the purposes of describing the prior art both terms may be employed; however, for the purposes conveying the teachings of our patent disclosure, applicants prefer the term GnRH and will use it in describing their compounds.
Be that as it may, some prior art chemical sterilization procedures are specifically adopted to alter luteinizing hormone secretion before the animal has attained the age of puberty. This is not surprising since the role of luteinizing hormone in sexual maturation is well known. Luteinizing hormone is a gonadotropic hormone found in the anterior lobe of the pituitary gland and, in male animals, it is known to stimulate the interstitial cells of the testes to secrete testosterone (see generally, The Merck Index, 8th edition, p. 560 (1968), Encyclopedia of Chemical Technology, Vol. 7, pp. 487-488 (1951)).
One approach has been to use certain chemicals to produce antibodies in an animal which exhibit cross-reactivity with the gonadotropins produced by the animal's pituitary gland. It is generally thought that with such early antigenic stimulation, formation of antibodies is more continuously stimulated by the release of endogenous hormones and that early immunization with such luteinizing hormone deters the maturation of the gonads and adnexal glands. This, in turn, is thought to inhibit spermatogenesis at the spermatogonial level. For example, U.S. Pat. No. 4,691,006 teaches injection of a compound having an amino acid sequence of at least 20 units for purposes of eliciting formation of antibodies which exhibit cross-reactivity with the gonadotropins produced by the animal's pituitary. With early antigenic stimulation of this kind, the formation of such antibodies is more continuously stimulated by release of endogenous hormones. Early immunization with such luteinizing hormone also deters the maturation of the gonads and adnexal glands. However, the art has also recognized that early immunization of this kind may tend to make the interstitial tissues fibroblastic. It has also been found that such early stimulation of the immunologic system leads to development of a high titered antiserum to luteinizing hormone which remains at relatively high levels. Nonetheless, periodic boosters of such compounds are often necessary even for adult animals sterilized before puberty in order to maintain high levels of the neutralizing antibodies.
Similarly, luteinizing hormone has been administered to animals after they have attained the age of puberty in order to atrophy their reproductive organs and to cause a decrease in libido (see generally, M. Tallau and K. A. Laurence, Fertility and Sterility, Vol. 22, No. 2, February 1971, pp. 113-118, M. H. Pineda, D. C. Lueker, L. C. Faulkner and M. L. Hopwood, Proceedings of the Society for Experimental Biology and Medicine, Vol. 125, No. 3, July 1967, pp. 665-668, and S. K. Quadri, L. H. Harbers, and H. G. Spies, Proc. Soc. Exp. Biol. Med., Vol. 123, pp. 809-814 (1966). Such treatments also impair spermatogenesis in noncastrated adult male animals by interruption of the spermatogenic cycle.
Other chemical sterilization agents have been specifically designed for use on female animals. For example, it is well known that certain antigens will produce an antiserum against a requisite estrogen. This is accomplished by first making an antigen and then injecting said antigen into an animal for purposes of antiserum production. The animal is then bled to recover the antiserum. Any female animal of the same species as the host animal may then be injected with the antiserum at the proper time prior to ovulation and the injected antiserum will cause temporary sterilization of that animal.
Other methods of chemical sterilization have been based upon direct chemical attack upon certain cells of the pituitary itself (as opposed to chemical attacks upon the hormone products of such cells) with a view toward permanently destroying such cells. Again, this approach is suggested by the fact that follicle stimulating hormone (FSH) and luteinizing hormone (LH) (sometimes referred to as gonadotropins or gonadotropic hormones) are released by the pituitary gland to regulate functioning of the gonads to produce testosterone in the testes and progesterone and estrogen in the ovaries. They also regulate the production and maturation of gametes.
Several chemical agents have been proposed for such purposes. However, it has been found that most chemical agents which are in fact capable of destroying the gonadotrophs of an animal's anterior pituitary gland also tend to produce extremely toxic side effects which can severely weaken, and sometimes kill, the treated animal. Hence, with respect to the general subject of chemical sterilization, it can be said that any chemical capable of producing sterilization without, or with minimal, toxic side effects would be of great value in the fields of animal husbandry, veterinary medicine and wildlife control.
To date, perhaps the closest concepts and/or compounds to those described in this patent disclosure are found in a publication by Myers, D. A., Murdock, W. J. and Villemez, C. L., entitled Protein-Peptide Conjugation By A Two-Phase Reaction: Biochem. J., 227:1 pg. 343 (1985). This reference teaches a sterilization procedure employing a GnRH analog comparable to that utilized by applicant in one of his more preferred GnRH/toxin conjugate compounds, namely one based upon a GnRH/diphtheria toxin conjugate. However, there are some very pronounced differences in the toxin portions of the respective molecules. These differences reside in the fact that different parts or portions of the diphtheria toxin are employed in the respective resulting compounds. More specifically, the conjugate reported by Myers et al. utilized only the toxin domain of the diphtheria toxin molecule while applicant's diphtheria toxins are characterized by their possession of the membrane translocation domain of this toxin as well as the toxic domain. The details and significance of these molecular differences are important to this patent disclosure and will be discussed at greater length in subsequent parts of this patent disclosure.
However, before leaving this discussion of the GnRH/diphtheria conjugate aspect of the prior art, it also should be noted that in addition to the article by Myers et al. noted above, Myers, on another occasion, published additional information concerning his diphtheria toxin-GnRH analog conjugate. This was done in his Ph.D. thesis at the University of Wyoming in 1987, entitled: "Hybrid toxins: An approach to cell specific toxicity." This thesis contains basically the same information as the above-noted 1985 publication, but--of course--in much greater detail. For example, the thesis includes further information on the biological activity of the Myers conjugate. A second part of this thesis addresses modifications of Myers' diphtheria toxin in a manner similar to that described above, but using further information published by Colombatti et al. in the Journal of Biological Chemistry 261:3030 (1986).
Another reference of possible interest in this regard was recently published in the INTERNATIONAL JOURNAL OF PHARMACOLOGY 76: R5-R8 by Singh et al. entitled "Controlled release of LHRH-DT from bioerodible hydrogel microspheres." Generally speaking, it teaches that a natural GnRH/diphtheria toxin can be used as a vaccine. In this case the LHRH-DT molecule induces production of antibodies to GnRH which then serve to inactivate endogenous LHRH in the circulation. Without the endogenous LHRH, there is no stimulation of the anterior pituitary gland to secrete LH and the gonads will cease functioning. However, as the antibody titers fall, endogenous GnRH will again stimulate the anterior pituitary gland, LH secretion and gonadal function will return. Here again, those skilled in this art will appreciate that this is an entirely different approach from the "direct chemical attack on the pituitary gland" approach taught in this patent disclosure. That is to say that--unlike Singh's antibody production approach--applicant's conjugate will not generate antibodies to GnRH and no neutralization of endogenous GnRH will occur. Instead, with applicant's approach, the cells in the anterior pituitary gland which are activated by GnRH will be destroyed by direct chemical attack thereon. Moreover, this attack results in permanent, rather than temporary sterility.
However, before going on to these details, it also should be noted that knowledge of the above noted sex hormone functions has produced several advances in the field of human medicine as well. For example, the potential for achieving chemical castration (rather than "surgical" castration) with certain luteinizing hormone-releasing hormone (LHRH) analogs has been reported (see for example, Javadpour, N., Luteinizing Hormone-Releasing Hormone (LHRH) in Disseminated Prostatic Cancer; 1M, Vol. 9, No. 11, November 1988). Table I below gives the structure of LHRH and the structure of certain analogs (e.g., Goserelin, Leuprolide, Buserelin and Nafarelin) of LHRH which are capable of temporarily suppressing luteinizing hormone secretion and thereby suppressing the gonads. As a consequence, these LHRH analogs have come to be regarded as a promising new class of agents for the treatment of various host-dependent diseases, especially prostatic cancer. In referring to Table I, it first should be noted that LHRH has a decapeptide structure and that substitution of certain amino acids in the sixth and tenth positions of the LHRH produce analogs which render agonists that are up to 100 times more potent than the parent LHRH compound (hence these compounds are often referred to as "superagonists"). The structures of LHRH and the most commonly known LHRH superagonists are listed below.
______________________________________ STRUCTURES OF LHRH AND SOME SUPERAGONISTS (Superagonists have substitutions at positions 6 and 10) ##STR1## SUPERAGONISTS: Name Subs. at 6 Subs. at 10 Terminator ______________________________________ Goserelin: D-Ser(tBu) AzaGly Amide Leuprolide: D-Leu des-Gly Ethylamide Buserelin: D-Ser(tBu) des-Gly Ethylamide Nafarelin: D-2-NaphthylAla None Amide ______________________________________
While these compounds represent the most promising means for palliative therapy because of their relative lack of side effects, they are particularly expensive and must be administered repeatedly. Even the newest formulations utilizing polymer encapsulated drug or other depot forms will require at least monthly administration. Improved depot forms also are presently in development, but they too are likely to be equally expensive and they too will probably require monthly administration. In response to these many drawbacks, applicants have developed a class of compounds which is capable of producing safe, inexpensive, chemical castration as an alternative to surgical castration. Such drugs also greatly simplify therapy of the generally elderly patients with prostate cancer, and could eliminate the need for surgical castration (still preferred by many urologists) as well as provide a medical alternative to oophorectomy in females with advanced breast cancer. Moreover, as a model system, the ability to eliminate pituitary gonadotrophs in vivo, which are regulated by GnRH receptors in response to ligand stimulation in a predictable fashion, is a highly appealing first step toward the more complex use of toxins conjugated to antibodies to eliminate tumor targets. Hence, use of applicants' compounds generally will fall into two major areas of use. The first is sterilization of mammals of all types; the second is chemical castration of mammals in general, and human beings in particular, for purposes of treating breast or prostate cancer by ablating those pituitary cells, namely gonadotrophs, responsible for LH secretion.